Method for protecting joints for electrical cables, protective coating for said joints and joints thus protected

ABSTRACT

A method for mechanically protecting a connection between at least two components forming part of an electrical and/or telecommunications network. At least one protective coating is provided, the coating being produced from an expanded polymeric material and providing the connection with a mechanical impact strength and ensuring a predetermined heat exchange between the connection and the external environment. This coating is axially and circumferentially continuous with respect to the connection. The present invention also relates to a coating for the mechanical protection of a connection between two components and to a joint thus protected.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/EP01/04508, filed Apr. 20, 2001, the content of which isincorporated herein by reference, and claims the priority of EuropeanPatent Application No. 00108780.8, filed Apr. 25, 2002, the content ofwhich is incorporated herein by reference, and claims the benefit ofU.S. Provisional Application No. 60/199,377, filed Apr. 25, 2000, thecontent of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for protecting a joint forelectrical cables, in particular for underground electrical cables, to aprotective coating intended to preserve the integrity of said joint onceinstalled and to a joint for electrical cables thus protected.

2. Description of the Related Art

Generally cables for conveying or supplying energy, in particular forconveying or supplying medium-voltage or high-voltage energy, comprise,from the inside towards the outside of the cable: a metal conductor, aninner semiconductive layer, an insulating layer, an outer semiconductivelayer, a metal screen—usually made of aluminium, lead or copper—and anexternal polymeric sheath. The predetermined sequence: metal conductor,inner semiconductive layer, insulating layer and outer semiconductivelayer is generally known by the term “cable core”.

In order to form a joint between two sections of electrical cable, forexample of the single-pole type, the ends of both said sections areprocessed beforehand so as to expose, over a portion of defined length,each of the aforementioned elements which make up the abovementionedcables.

Subsequently, the joining operation consists in forming an electricalconnection between the conductor elements, for example by means ofsoldering or scarfing of the latter, and positioning, in the zone wherethe said conductor elements are joined together, an elastomeric sleeveconventionally known by the term “joint”.

Generally, said sleeve has a form which is substantially cylindrical inits central portion and of frustoconical type at its ends so as toprovide an optimum connection between the cable sections being joinedand the joint itself.

This sleeve consists of a plurality of radially superimposed elementsintended to restore the electrical and mechanical connection betweeneach exposed layer of a first cable section and the correspondingexposed layers of a second cable section.

Therefore, starting from its innermost portion, said sleeve generallycomprises: a voltage distribution layer made of material with a highdielectric constant next to the insulating layers of the cable, a layerof insulating material of considerable thickness which surrounds saidvoltage distribution layer, and a layer of semiconductive materiallocated radially on the outside of said insulating layer and suitablyconnected to the outer semiconductive layer of each cable sectiondesigned to restore the continuity of the outer semiconductive layers ofsaid first and second section. Generally, the zone where the twoconductor elements are joined together is filled with anelectrical-field control material.

Methods for constructing joints known in the art are described, forexample, in documents EP-379,056; EP-393,495; EP-415,082; EP-199,742;EP-422,567 in the name of the Applicant.

Generally, this sleeve is produced separately and supplied fitted, in anelastically-dilated condition, on a hollow tubular support made of rigidplastic. The sleeve thus supported is engaged around one of the sectionsduring a step preceding the formation of the joint between the metalconductors.

This support may be constructed using different methods which allow theremoval thereof once the abovementioned joint has been formed. Forexample, the tubular support may be obtained from a strip-like elementhelically wound so as to form a plurality of adjacent spirals fastenedtogether so that, when a pulling force is exerted on a free end portionof said strip-like element, the tubular support is able to collapse, dueto gradual separation of the spirals, and allow correct positioning ofthe sleeve. In so doing, the sleeve elastically contracts, gripping thecable sections in the joining zone. This sleeve is of thecold-retractable type. Embodiments of said supports are described, forexample, in the documents EP-541,000; EP-735,639; EP547,656; EP-547,667in the name of the Applicant.

Alternatively, the sleeves may be made using heat-shrinkable materials,thus producing the so-called heat-shrinkable sleeves described, forexample, in the patent U.S. Pat. No. 4,383,131.

Generally a joint also comprises an element intended to restore themetal screen, such as, for example, a tin-plated copper strip which isapplied starting from the exposed metal screen portion of the firstsection and terminating on the exposed metal screen of the secondsection.

In the case where the joining operation is performed between twosections of electrical cable of the multi-pole—for example double-poleor triple-pole-type, the procedure described hitherto is repeated foreach single phase of each cable.

Finally, a joint as defined further above normally also comprises anexternal polymeric sheath suitable for restoring the external mechanicalprotection of the cable and fitted in the joining zone, in a positionradially on the outside of the aforementioned sleeve.

Generally, this sleeve is intended to protect the underlying elements ofthe joint from coming into contact with moisture and/or water from theoutside.

Said sheath may be of the heat-shrinkable type or cold-shrinkableelastic type or may be obtained by means of a strip-forming step, whichmay also be combined with the use of suitable mastic sealants.

This sheath is inserted on one end of one of the said cable portionsduring a step preceding both positioning of the tubular support carryingthe abovementioned sleeve and formation of the connection between theconductor elements.

In accordance with further operating methods, restoration of theexternal mechanical protection of the cable may also be achieved usingseveral sheaths, for example three in number, arranged so that one pairof sheaths is fitted onto the aforementioned frustoconical portions ofsaid joint and a further sheath is fitted onto the substantiallycylindrical portion of the latter.

Generally, the zone where two cables for conveying or supplying electricenergy are joined together inevitably forms a discontinuity in theconveying or supply network and, consequently, a weak point in thelatter, also in view of the complexity of the aforementioned joiningzone.

This complexity is due, in fact, both to the plurality of operationswhich must be carried out by the technical personnel responsible forinstallation of the joint and to the structure itself of the joint inthat its composition, as regards its main components, is as describedfurther above.

In order to ensure a high degree of mechanical protection, guaranteeingoptimum and long-lasting operation, the joints are generally provided onthe outside of their structure with a protective coating having asuitable form and made of suitable materials, which enclose the joiningzone internally.

It must be emphasized, in fact, particularly if the cables arepositioned, as in most cases, in trenches dug in the ground, thatinevitably the joints themselves also have to be arranged in positionand made operative inside the said trenches.

However, the latter represent an environment which is difficult tocontrol since, owing to their nature they have restricted dimensions,accumulated debris is present along the edges of the excavations and thetechnicians preparing the joint move around and operate within them.

Under such working conditions it frequently happens that the debrisand/or work equipment used by said technicians may accidentally strikethe external surface of the joints and cause, for example, deformationsin the layer of insulating material forming part of the latter.

These deformations are particularly undesirable since they cause areduction in the insulating capacity of said layer, as well asseparation of the latter from the semiconductive layer, thereby givingrise to partial discharges, resulting in irreversible damage to thejoint.

Known systems for protecting the joints from the environment surroundingthem, in particular from dust and moisture, envisage, for example, theuse of particularly simple containers which use rapid-closure systems,for example of the bayonet type as described, for example, in the patentU.S. Pat. No. 4,684,764.

Devices known in the art and designed to provide the joints withprotection of the mechanical type, for example against accidental knockswhich, as mentioned, may occur during laying and/or installation,consist, for example, of rigid containers positioned outside the saidjoints.

Generally, said containers are divided into two halves which are formedso as to be arranged around the joining zone and provide it with thedesired protection. Moreover, they are often made of metallic material,for example aluminium, coated externally with an anti-corrosive paint.

Said paint has the function of avoiding, or at least limiting, thedevelopment of any corrosive phenomena which, locally deteriorating theexternal surface of said containers, in addition to weakening themechanical strength thereof, would allow the undesirable infiltration ofmoisture and/or water inside the joint.

Generally, these containers have dimensions greater than those of thejoint to be protected since, on the one hand, it is unfeasible from aneconomic point of view to produce containers with specific dimensionsfor each type of joint and cable to be joined and, on the other hand,during assembly, it is necessary to ensure that there is sufficientlyample room for manoeuvre to perform correct and rapid positioning of theprotective container on the said joint.

Generally, a filling material is introduced inside said containers,namely into the gap between the external surface of the joints and theinternal walls of the containers, said filling material performing thefunction of providing a protective layer against any accidental knocksaffecting the joint and providing the joint protection system with agreater mechanical strength. If necessary, said filling material is alsochosen so as to form a barrier against the infiltration of moistureand/or water from the outside.

Generally, the filling material which is used is a thermosetting resinsuch as, for example, an epoxy, polyurethane or similar resin.

The document GB-1,497,051 describes a further mechanical protectiondevice for cable joints, consisting of a heat-shrinkable elastomericsleeve, the internal surface of which is coated with a plurality ofreinforcing elements of elongated shape, arranged parallel to thelongitudinal axis of said sleeve.

Said reinforcing elements are generally in the form of wires, bars orstrips of metallic, plastic or fibreglass material, which are kept inposition adjacent to each other, for example by means of an adhesive, amastic or a support sheet.

The document EP-093,617 relates to a further mechanical protectiondevice for electrical cable joints, comprising a set of elongatedelements which are kept adjacent to each other on the external surfaceof the joint and a hot-shreankable or cold-shreankable sleeve designedto be positioned around said set.

These elements, which are preferably made of metallic material, arefastened to each other so as to form a kind of cage structure by using,for example, cords, hooks, soldering zones, support sheets provided withan adhesive element or flexible strips.

Owing to the presence of said plurality of elements, this assembly isable to follow the profile of the joint where there are changes in itscross-section, reducing the overall dimensions of the coupling betweenprotection device and joint.

In order to ensure that such a result may be achieved, each of the saidelements is formed so as to have a substantially straight progressionalong the substantially cylindrical portions of the underlying joint anda diverging or converging progression where the cross-section of thejoint respectively becomes thicker or thinner.

The abovementioned shreankable sleeve, which may also not be present,generally has a longitudinal extension greater than that of saidelements so that the sleeve may make contact with a cable portionupstream of the joint and a cable portion downstream of the joint,sealing off the latter from the surrounding environment.

The Applicant has noticed that the protection devices for jointsaccording to the known art have a plurality of drawbacks.

For example, in order to ensure a satisfactory mechanical impactstrength, in the case where said device is in the form of a containerlocated on the outside of the joint, generally this container is made ofa material sufficiently rigid to safeguard the joint contained therein,for example metal or plastic material.

However, this feature is viewed as being particularly unfavourable sincethis material, being rigid, does not allow damping of an impactresulting from, for example, excavation debris falling inside the trenchwhere the cable is laid, as the energy contained in said debris istransferred practically entirely onto the underlying joint.

Moreover, if this impact is particularly violent, it may cause permanentdeformation of the protective container, resulting in it beingineffective in the damaged zone in the event of any renewed accidentalimpacts in the same zone and resulting in continuous crushing of theelements of the joint, adversely affecting the operation thereof.

As mentioned further above, in the case where the protective containeris of the metallic type, it is generally coated with an anti-corrosivepaint so as to prevent the occurrence of corrosive phenomena due toattack by water and/or moisture inevitably present in the ground.

However, the use of this anti-corrosive paint does not eliminateentirely this risk since any debris falling onto the external surface ofthe protective container inevitably forms incisions on the latter, evenof a limited nature, resulting in removal of the paint.

These zones, therefore, constitute areas favouring the development ofcorrosion which, in particularly favourable environmental conditions,may develop rapidly and adversely affect the protective capacity of thecontainer.

Containers of the metallic type do not have, moreover, any flexibilityin the longitudinal direction, this aspect making installation thereofin an operative condition less easy.

In order to prevent water and/or moisture from spreading towards thejoint to be protected, as mentioned further above, some solutions of theknown art envisage using a filling material to be positioned in the gapbetween the container and the external surface of the joint. However,the use of this filling material has a few drawbacks.

A first drawback consists in the complexity of the protective systemsince the installation process envisages a first step for arranging thecontainer around the joint and a next step during which the fillingmaterial is introduced into the space between joint and container andsaid material is left to harden or made to harden for example by usingheat.

This means, therefore, that the operation involving preparation of theprotective coating of the joint is fairly complex and requires a fairlylong assembly time and the use of qualified technical personnel—theseaspects obviously resulting in a substantial increase in theinstallation costs.

A further drawback consists in the fact that the structure of theprotective container is necessarily more complex since it is necessaryto provide at least one inlet opening for the coating material, a devicefor closing the said opening as well as a sealing system both for theinlet opening and for the connection zone between the two half-shellswhich generally form the abovementioned protective container.

Moreover, if the filling material should be a thermosetting resin, as isthe case in nearly all installation processes, this aspect constitutes afurther disadvantage due to the nature itself of this resin. Handling ofthe latter, in fact, generally requires the use of suitableprecautionary measures and a considerable degree of care since saidresins are irritants (to the skin, eyes or respiratory tract) and, insome cases, are even toxic.

It is necessary to point out, moreover, that the use of a protectivecontainer according to the known art has the further drawback that itrequires an operation involving joining together of the two halvesforming it, said operation being performed manually and directlyon-site, namely within the laying trench and therefore in precariousconditions, with limited freedom of movement. All this results ingreater complexity of operation and an inevitable delay in theinstallation time.

The technical solution described in the document EP-093,617 referred toabove goes beyond the traditional concept of a protective device in theform of a container and suggests the use of a device comprising a set ofelongated elements and a retractable sleeve to be arranged around thelatter.

However this solution, which is based on a concept different from thatof the prior art, also has certain drawbacks.

A first drawback, which is particularly significant for the purposes ofachieving an acceptable mechanical strength of the joint, consists inthe fact that the type of combination of the above-mentioned elongatedelements does not allow the formation of a continuous protective layerable to ensure the same level of protection against impacts over thewhole external surface of the joint.

In fact, these elements are arranged alongside each other in thelongitudinal direction, parallel to the longitudinal direction of thejoint, without forming a continuous protective layer on the outside andover the circumference of the joint.

A further disadvantage of the device according to the documentEP-093,617 consists in the fact that the elongated elements which formit are made preferably of metallic material or of moulded plasticmaterial.

As mentioned further above with reference to the containers of the knownart, the use of a metallic material for protective purposes is dislikedsince the device is excessively rigid and is unable to dampen theimpacts to which it may be subject, transferring practically entirelythe impact energy onto the underlying layers.

Moreover, the choice of these materials results in the protective devicebeing particularly heavy, thereby aggravating the working conditions ofthe technical personnel responsible for joining the cables.

Furthermore, plastic materials in general do not have a high resistanceto violent impacts unless special polymer products are used.

A further problem which the solutions of the known art are unable tosolve in a satisfactory manner consists in the disposal of the heatwhich is produced inside a joint following the passage of electriccurrent. In fact, should said heat not be adequately disposed of, a hotpoint is formed in the distribution system, said hot point consisting ofthe joint itself. This fact results in an undesirable reduction in thecurrent flow rate inside the cable.

In order to ensure an at least partial disposal of said heat, thesolution of the known art relating to a container filled with fillingmaterial requires that the thickness of said material should besufficiently small. However, if this thickness is particularly small,the mechanical strength of the protective coating is inevitablyweakened.

The Applicant has therefore established the need to provide a mechanicalprotection for electrical cable joints which is able to guarantee a highmechanical impact strength, with particular reference to theinstallation of underground electrical lines, and an optimum disposal ofthe heat in the joining zone, this protective coating having a reducedthermal resistance, not being affected by particular problems oftoxicity and/or handling and not influencing negatively the weight andthe thickness of the joint/protective coating assembly.

The Applicant has perceived, moreover, that there is a need to devise amethod for protecting joints which can be implemented in a simple mannerand with little effort by the operator and which does not requirecomplex operations, resulting in advantages both in terms of the speedof installation and in terms of lower costs.

SUMMARY OF THE INVENTION

The Applicant has found that this objective is achieved by providing onthe external surface of the joint a protective coating made of expandedpolymeric material with a predefined thermal resistance.

Therefore, according to a first aspect thereof, the present inventionrelates to a method for mechanically protecting a connection between atleast two components forming part of an electrical and/ortelecommunication network, comprising the step of providing at least oneprotective coating around said connection, characterized in that saidcoating is produced using an expanded polymeric material suitable forproviding said connection with a mechanical impact strength and, at thesame time, ensuring a predetermined heat exchange between saidconnection and the external environment.

In accordance with a first embodiment, said connection is enclosedinside said protective coating.

According to a further embodiment, said connection is helically woundwith said protective coating.

In a further embodiment, said protective coating is obtained by linkingtogether a plurality of separate elongated bodies arranged around saidconnection.

According to a second aspect thereof, the present invention relates to acoating for mechanically protecting a connection between at least twocomponents forming part of an electrical and/or telecommunicationnetwork, characterized in that said coating, arranged in a positionradially outside said connection, is made of an expanded polymericmaterial and both provides said connection with a mechanical impactstrength and ensures a predetermined heat exchange between saidconnection and the external environment.

Preferably, said coating is axially and circumferentially continuouswith respect to said connection.

This coating is in the form of a sheet or is in tubular form or is ofthe modular type, comprising, in this latter case, a plurality ofseparate elongated bodies linked together around said connection.

According to a third aspect thereof, the present invention relates to ajoint for electrical cables designed to convey or supply energy, saidjoint comprising:

-   -   at least one electrical connection between a conductor of a        first electrical cable and a conductor of a second electrical        cable;    -   at least one electrical insulating layer arranged in a position        radially outside said connection, and    -   a protective coating arranged in a position radially outside        said electrical insulating layer, characterized in that said        coating is made of an expanded polymeric material and is        suitable for providing said connection with a mechanical impact        strength and, at the same time, ensuring a predetermined heat        exchange between said connection and the external environment.

BRIEF DESCRIPTION OF THE DRAWINGS

The description, provided hereinbelow, relates to the accompanyingdrawings provided solely by way of explanation and not intended to belimiting in any way, where:

FIG. 1 shows a partially axially-sectioned, schematic side view of thejoining zone of two single-pole electrical cables according to anembodiment of the known art;

FIG. 2 shows a partially-sectioned side view of a type of protectivecoating according to the known art;

FIG. 3 shows a perspective view of an embodiment of the protectivecoating according to the present invention;

FIG. 4 shows a perspective view of an elongated element according to afurther embodiment of the present invention, and

FIG. 5 shows a cross section through a protective coating formed by acontinuous succession of elongated elements of the type illustrated inFIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The description which follows and the figures associated therewithillustrate the case where the protective coating and method according tothe present invention relate to an electrical connection between twoelectrical cables for conveying or supplying electric energy.

It must be emphasized, however, that generally this protective deviceand method may be applied to any electrical or optical connectionforming part of an electrical and/or telecommunication network.

Moreover, the present invention may advantageously be used also as amechanical protection system for a zone where there is a connectionbetween a cable and any apparatus.

As will emerge more clearly from the present description, whichconcentrates on a zone where two electrical cables are joined together,the structural aspects characteristic of the numerous types of jointwhich may be operationally realized will not be described in greatdetail since they are outside the scope of the present invention, forthe purposes of which the type of joint considered does not constitutean aspect limiting the said invention.

In FIG. 1 reference sign 10 denotes generically a joint according to anembodiment of the known art, intended for the electrical connection of apair of cables 11, 12 of the single-pole type.

As already mentioned, this joint is obtained by arranging, coaxiallyfacing each other, the ends of said cables 11, 12 progressively strippedof their associated coating layers forming part of the respectiveconductors 13, 14 which are exposed over a predefined section.

The coating of said cables 11, 12 is removed by exposing, for each cableand over a given length, in succession the insulating layer 15, 16, theouter semiconductive layer 17, 18, the metallic screen 19, 20 and theexternal polymeric sheath 21, 22.

As shown, the exposed end portions of each conductor 13, 14 areelectrically connected together by means of an element 23 which is knownper se and consists, for example, of a soldering zone or a suitablescarfing system.

Once the electrical connection between the abovementioned conductors 13,14 has been performed, the space corresponding to the removed sectionsof insulating material 15, 16 is filled with a deformable,field-control, filling material 24 which is known per se.

The joining zone is covered by a sleeve 25, made of elastic material,for example a cross-linked polymeric material, which is slidably fittedonto one of the cables 11, 12 before they are connected together andpositioned above said joining zone once said element 23 and the fillingmaterial 24 have been applied.

Said sleeve 25 is freed above the joining zone using knowntechniques—for example by means of a removable support element, asmentioned above—so as to form a coating covering the exposed sections ofthe insulating layer 15, 16.

Alternatively, the sleeve 25 may consist of a heat-shrinkable material.

Additional elements 26, which are also well-known to persons skilled inthe art, are arranged so as to line the ends of said sleeve 25 andrestore the continuity of the screen, said additional elements 26 beingconnected to the external semiconductive layer 17, 18 of the two cables11, 12.

In FIG. 2 the reference number 30 denotes overall a joint 31 for exampleof the type shown in, FIG. 1, provided with a protective coating 32according to the known art.

In greater detail, the joint 31 is partially sectioned so that the pairof cables 11, 12 being joined together are visible, said cables, unlikethat shown in FIG. 1, being of the three-pole type. The cores of saidcables have been indicated overall by the reference numbers 33, 34, 35.

In FIG. 2 it is possible to distinguish, moreover, the binding elements36, 37 generally consisting of a reinforced adhesive tape which keeps,joined together, the individual phases in the central joining zone andthe external sheath 38, for example of the heat-shrinkable type coatingthe joint 31.

The protective coating 32 comprises an external container 39—called“muffle” in technical jargon—which is preferably made of metallicmaterial and which receives, inside it, filling material 40 which, asmentioned further above, is generally a thermosetting resin.

In FIG. 3 the reference number 50 denotes overall a joint according tothe invention comprising the joint 31 according to FIG. 2 and aprotective coating 51 according to an embodiment of the presentinvention.

In accordance with the embodiment shown, said protective coating 51 isin sheet form and can be wound in a position radially outside the joint31, as indicated by the arrow A in FIG. 3, enclosing inside it the saidjoint.

Said protective coating 51 therefore encloses the joint 31, closelyfollowing the external profile and adapting to variations in its crosssection in the zone where the two electrical cables are joined together.

In the embodiment shown in FIG. 3, the protective coating 51 ispositioned beneath the external sheath 38 (not shown in FIG. 3) which,as mentioned, is conventionally arranged in a position radially outsidethe joint 31 on completion of the latter.

In this case, therefore, once in position, the protective coating 51 iskept in the correct operating condition by the external sheath 38 whichcovers it.

In accordance with a further embodiment (not shown), the protectivecoating 51 is arranged above the external sheath 38 in a manner similarto that shown in FIG. 2 with reference to the muffle 39.

In this case, said protective coating 51 is kept in position by using asuitable fixing system, for example by arranging adhesive tape along itscircumferential extension and at predetermined distances along itslongitudinal extension.

In accordance with a further embodiment (not shown), the protectivecoating 51 is obtained by providing at least two successive windings ofsaid continuous sheet so as to define at least one pair of superimposedcontinuous layers. This embodiment may be envisaged where, inparticularly critical operating situations, it is necessary to ensure aparticularly high mechanical strength.

With reference to FIGS. 4 and 5, a protective coating 60 according to afurther embodiment of the present invention is shown.

Said protective coating 60 (shown in FIG. 5) comprises a plurality ofelongated bodies 52 which are arranged substantially parallel to eachother and coaxial with respect to the axis of the cables to be joinedtogether, being linked so as to define the above-mentioned protectivecoating 60 in a position radially outside said joint 31.

For the purposes of the present description and the following claims,the term “continuous protective coating” is understood as meaning auniform and uninterrupted protective coating, both in the axialdirection and in the circumferential direction, over the whole extensionof the joining zone. This means that there are no portions of saidjoining zone—even with a limited extension—devoid of said protectivecoating.

Therefore, in accordance with the present invention, the protectivecoating is not necessarily made in the form of a continuous sheet 51,but may also be obtained by linking together a plurality of separatebodies 52 provided that this linkage defines, in any case, a protectivecoating of the continuous type which completely coats the externalsurface of the joint.

As already mentioned, FIG. 4 shows a perspective view of a particularembodiment of said elongated bodies 52, linking together of whichresults in the formation of the protective coating 60, thecircumferential extension of which is indicated by the broken lines inFIG. 5.

Said elongated bodies 52, viewed in cross section, have a shapesubstantially in the form of a Y which has proved to be particularlyadvantageous for the purposes of rapid, simple and efficient linkingtogether of said bodies.

In greater detail, the diverging and slightly rounded sections 53, 54 ofsaid Y define a curved profile mating with the base section 56 of saidY.

In this way, therefore, the external curved profile 57 of the basesection 56 of a given elongated body 52 is able to mate with the curvedprofile 55 of a body 52 preceding it, with reference to the direction ofengagement indicated by the arrow B in FIG. 5.

Moreover, the longitudinal extension of said bodies 52, in the directionX of FIG. 4, is such that the linking together of said bodies allows thejoining zone to be coated over its whole extension.

Each base section 56, once inserted between the diverging sections 53,54 of a body 52 adjacent to it, forms an articulation point for thecircumferential extension of the protective coating 60 since the curvedprofile 55 referred to above is partially free to slide along theabovementioned external curved profile 57 of the base section 56.

As can be clearly understood from FIG. 5, said sliding action is onlypartial since the ends of the diverging sections 53, 54 cannot bedisplaced beyond the zones 61 where said diverging sections 53, 54 arejoined to said base section 56.

In accordance with this embodiment, the protective coating 60 has aparticularly advantageous modular feature since it may be adapted easilyand rapidly to any type and size of joint.

In FIG. 5, the joint 31, which is illustrated with a protective coating60 in a position radially on the outside thereof, obtained by means ofthe sequential arrangement of the plurality of elongated bodies 52, isnot shown in detail and the portion shown in broken lines in the figureindicates generally its external circumferential dimensions in a crosssection thereof.

The continuous protective coating 60 according to the present embodimentmay be obtained, for example, by inserting with pressure the portion 58of a first body 52, namely the portion corresponding to the base section56 of the Y, into the space defined by the portions 59, namely theportions corresponding to the diverging sections 53, 54 of said Y, of asecond body 52 which precedes/follows said first body.

In accordance with a further embodiment, the portion 58 of a first body52, instead of being pushed with pressure into the space defined by theportions 59 of the second body 52, is inserted into said space by meansof simple sliding.

As can be seen from FIG. 5, the particular geometry of the elongatedbodies 52 allows the formation of a protective coating 60 whichsurrounds entirely the external surface of the joint 31.

Therefore, if debris should accidentally strike the joint 31, theprotective coating 60 according to the present invention ensurescontinuous protection also in the zones for connection of a body 52 withthe bodies which precede it and follow it, respectively.

The particular geometry of the elongated body 52 shown in FIG. 4constitutes one of the possible solutions which may be adopted in orderto—form a protective coating 60 of the modular type. For example, thediverging arms 53, 54 of the elongated body 52, instead of having ajoining profile 55 of the curved type, may have a profile in the form ofa dovetail or an arrow. Similarly the base section 56 of said Y mustalso be provided with a configuration mating with said diverging arms.

The protective coating according to the present invention is installedin a simple and rapid manner directly on-site, namely inside theexcavation trench, by the personnel performing the joining operation.

In the case where said protective coating is in sheet form (as shown inFIG. 3), it is wound around the joint using the methods describedfurther above.

In a further embodiment, said sheet does not have a length equal to thelongitudinal extension of the joint and a width at least equal to thecircumferential profile of the latter so as to cover the joint with asingle winding, but has dimensions such that it may be wound helicallyaround said joint forming a sort of taped arrangement with predeterminedpartial overlapping of its edges.

If, however, said protective coating is formed using a plurality ofelongated bodies, the latter are joined together, as already mentioned,directly in position, being of a number such as to cover thecircumferential extension of the joining zone.

In this embodiment, completion of the protective coating may be achievedby joining together the first and the last of said elongated bodies, ifnecessary forming a circumferential extension slightly greater than thatof the joint, or by providing an overlapping piece between the firstbody and the last body, fixing said overlapping piece by means ofsuitable fixing elements, for example adhesive tape.

The protective coating according to the present invention is thereforeable to achieve a dual aim: provide the joint with a system formechanical protection against knocks and ensure correct transfer of heatbetween the joint and the external environment in order to prevent theformation of a hot spot—i.e. the joint itself—inside the system forconveying or supplying the electric energy.

As regards the first of the above-mentioned aims, as already mentionedduring the course of the present description, a joint for electricalcables, in view of the particularly difficult installation environment,requires a protective coating which preserves the structural integrityof said joint both during the installation phase, which is particularlycritical, and during operation of said joint.

The protective coating according to the present invention, owing to thetype of material used and the geometrical design developed, ensuresoptimum mechanical protection while maintaining minimal weight anddimensions, as will be explained more clearly in the following of thepresent description.

As regards the second of the above-mentioned aims, it has to beremembered that, in general terms, the flow of an electric currentinside a conductor inevitably results in heating of the said conductorwhich is proportional to the square of the intensity of said current.Therefore, after the flow of said current, a rise in temperature occursinside the cable.

Consequently, for the same cross section of the conductor and on thebasis of the desired current intensity of the flow within the system,the task of the designer is to choose suitably the material of theinsulating layer of the cable so that, to avoid deterioration in theelectrical/mechanical characteristics of said layer, thistemperature—i.e. the maximum permissible temperature—is not exceeded.

Moreover, this increase in temperature also depends on the thermalresistance of the cable/joint/protective coating system, which must besufficiently small so that there is a suitable heat exchange with theexterior and the abovementioned maximum permissible temperature is notexceeded.

Generally, in a joint for electrical cables, in operating conditions,the situation which is created when electric current flows inside theconductors is similar to that described above with reference to a cable.

However, the thickness of the insulating layer of a joint must begreater than the corresponding thickness of the insulating layers of thecables being joined together.

This necessity is dictated by the fact that, unlike a cable which ismanufactured in a controlled environment and using a continuous process(for example extrusion), a joint is prepared directly on-site andrequires a significant amount of manual labour for its preparation.

In the case of a joint, therefore, the working environment is notcontrolled—for example is contaminated by dust and moisture—and,moreover, the joining operations are performed manually and,consequently, less precisely and with a high risk of contamination ofthe materials forming the said joint.

It has to be emphasized that this contamination is particularly harmfulsince it produces a deterioration in the electrical properties of theinsulating layer, with a consequent reduction in the value of thepermissible electrical stressing of said layer.

Therefore, in order to ensure a satisfactory safety margin, thethickness of the insulating layer of a joint is generally increased withrespect to the thickness of the insulating layer of a cable.

However, since, as emphasized further above, the thermal resistanceincreases with the increase in the thickness, as regards the above, theinsulating layer of the joint has, for the same material used, a thermalresistance which is greater than that of the insulating layer of theelectrical cables in the cable portions upstream and downstream of thejoining zone.

The result of this is that, following the flow of electric current andthe consequent above-mentioned temperature rise, the temperaturedifference between conductor and external environment is greater in thejoint than in the cables. In other words, for the same current flowingwithin the conductor, the joint is subject to a greater degree ofheating than the cables. This aspect is particularly critical since, asalready mentioned, should the maximum permissible temperature of theinsulating material of the cable be reached within the joint, in orderto prevent a deterioration in the electrical and mechanicalcharacteristics of the latter, it would be necessary to decrease thecurrent density of the line and, consequently, the current-carryingcapacity inside the system.

This effect is further accentuated if the joint is provided with amechanical protection system which does not allow optimum heat exchangebetween conductor and external environment.

In this case, in fact, the temperature rise inside the joint would besuch as to impose an unacceptable limit on the current-carrying capacityof the system.

In order to ensure a satisfactory heat exchange, as mentioned furtherabove, a filling material with a low thermal resistivity is introducedinto the protective containers for joints of the known art.

However, the thickness of this filling material may not be reducedexcessively since the external protective container, which is generallymade of metallic material, is unable to dampen the impacts which aretransmitted, almost entirely, to the underlying layers.

A large thickness of said filling material on the one hand, therefore,allows greater damping of the impacts, but on the other hand results inan increase in the thermal resistance of the system—owing to theincreased thickness—and greater overall dimensions of thejoint/protective container assembly.

The protective coating made of expanded polymeric material according tothe present invention ensures, however, excellent mechanical resistancewhich is effective over the whole external surface of the joint—owing tothe continuity of the coating—while allowing extremely small thicknessesof the coating which allow a reduction in the thermal resistance of thesystem as a whole. In this way, the temperature of the cable inside thejoint does not exceed the maximum permissible temperature of the cablein portions far from the joint and, consequently, the joint does notimpose a limit on the current-carrying capacity of the system.

In accordance with the present invention, the protective coating has athickness of between 3 mm and 25 mm, preferably between 3 mm and 15 mm,more preferably between 3 mm and 10 mm.

The protective coating according to the present invention is producedfrom an expanded polymeric material, this term being understood asmeaning a polymeric material having a predetermined percentage of “free”space within the material, namely a space not occupied by the polymericmaterial, but by gas or by air.

Generally, this percentage of free space in an expanded polymer isexpressed by means of the so-called “degree of expansion” (G), definedas follows:G=(d₀/d_(e)−1)*100

where d₀ indicates the density of the non-expanded polymer and d_(e)indicates the apparent measured density of the expanded polymer.

The protective coating made of expanded polymeric material according tothe present invention is obtained from an expandable polymer optionallysubjected to cross-linking, following expansion, as indicated in greaterdetail in the continuation of the present description.

This expandable polymer may be chosen from the group comprising:polyolefins, copolymers of various olefins, unsaturated ester/olefincopolymers, polyesters, polycarbonates, polysulphones, phenolic resins,ureic resins, and mixtures thereof. Examples of suitable polymers are:polyethylene (PE), in particular low density PE (LDPE), medium densityPE (MDPE), high density PE (HDPE) and low-density linear PE (LLDPE);polypropylene (PP); ethylene-propylene elastomeric copolymers (EPM) orethylene-propylene-diene terpolymers (EPDM); natural rubber; butylrubber; ethylene/vinyl ester copolymers, for example ethylene/vinylacetate (EVA); ethylene/acrylate copolymers, in particularethylene/methyl acrylate (EMA), ethylene/ethyl acrylate (EEA),ethylene/butyl acrylate (EBA); ethylene/α-olefin thermoplasticcopolymers; polystyrene; acrylonitrile-butadiene-styrene (ABS) resins;halogenated polymers, in particular polyvinyl chloride (PVC);polyurethane (PUR); polyamides; aromatic polyesters, such aspolyethylene terephthalate (PET) or polybutylene terephthalate (PBT);and their copolymers or mechanical mixtures thereof.

Preferably, the polymeric material is a polyolefinic polymer orcopolymer based on ethylene and/or propylene and in particular chosenfrom:

(a) copolymers of ethylene with an ethylenically unsaturated ester, forexample vinyl acetate or butyl acetate, in which the quantity ofunsaturated ester is generally between 5% and 80% by weight, preferablybetween 10% and 50% by weight;

(b) elastomeric copolymers of ethylene with at least oneC₃–C₁₂-α-olefin, and optionally a diene, preferably ethylene-propylenecopolymers (EPR) or ethylene-propylene-diene (EPDM) having preferablythe following composition: 35–90 mol % of ethylene, 10–65 mol % ofα-olefin, 0–10 mol % of diene (for example 1,4 hexadiene or5-ethylidene-2-norbornene);

(c) copolymers of ethylene with at least one C₄–C₁₂-α-olefin, preferably1-hexene, 1-octene and the like, and optionally one diene, generallyhaving a density of between 0.86 g/cm³ and 0.90 g/cm³ and the followingcomposition: 75–97 mol % of ethylene, 3–25 mol % of α-olefin, 0–5 mol %of a diene;

(d) polypropylene modified with ethylene/C₃–C₁₂-α-olefin copolymers, inwhich the ratio by weight between polypropylene and theethylene/C₃–C₁₂-α-olefin copolymer is between 90/10 and 30/70,preferably between 50/50 and 30/70.

For example, the commercial products Elvax® (Du Pont), Levapren®(Bayer), Lotryl® (Elf-Atochem) are included in class (a), the productsDutral® (Enichem) or Nordel® (Dow-Du Pont) in class (b), and theproducts Engage® (Dow-Du Pont) or Exact® (Exxon) in class (c), whilepolypropylene modified with ethylene/α-olefin copolymers may be found onthe market under the trade names Moplen® or Hifax® (Montell), orFina-Pro® (Fina), and the like.

In class (d), particularly preferred are the thermoplastic elastomerscomprising a continuous matrix of a thermoplastic polymer, for examplepolypropylene, and small particles (generally having a diameter of theorder of 1–10 μm) of a vulcanized elastomeric polymer, for examplecross-linked EPR or EPDM, dispersed in the thermoplastic matrix. Theelastomeric polymer may be incorporated in the thermoplastic matrix inthe non-vulcanized state and therefore dynamically cross-linked duringthe process through the addition of a suitable quantity of across-linking agent. Alternatively, the elastomeric polymer may bevulcanized separately and subsequently dispersed in the thermoplasticmatrix in the form of small particles. Thermoplastic elastomers of thistype are described, for example, in the documents U.S. Pat. No.4,104,210 or EP-324,430.

Among the polymeric materials particular preference is given to a highmelt strength polypropylene, as described, for example, in the patentU.S. Pat. No. 4,916,198, commercially available under the trade namesProfax® and Higran® (Montell S.p.A.). This document describes a processfor the production of said polypropylene by means of a step involvingirradiation of a linear polypropylene performed by means of ahigh-energy ionizing radiation and for a period of time sufficient tocause the formation of a significant quantity of long branches in thechain, said step being followed upon completion, moreover, by a suitabletreatment of the irradiated material so as to deactivate substantiallyall free radicals present in the irradiated material.

Even more preferably, among the polymeric materials particularpreference is given to a polymer composition comprising theabovementioned polypropylene with a high degree of branching, in aquantity generally of between 30% and 70% by weight, in a mixture with athermoplastic elastomer of the type belonging to class (d) mentionedabove, in a quantity generally of between 30% and 70% by weight, saidpercentages being given relative to the total weight of the polymercomposition.

The protective coating according to the present invention may beproduced by means of different techniques, for example by means ofmoulding or extrusion.

A moulding operation is preferred, for example, if the protectivecoating according to the invention is in the form of a sheet to be woundaround the joint to be protected.

If, however, said protective coating is obtained by linking together aplurality of elongated bodies, the latter are preferably formed by meansof extrusion and subsequently cut to size.

By means of extrusion it is also possible to produce a protectivecoating in tubular form (further embodiment not illustrated) which doesnot require any winding operation, but which must be inserted beforehandonto one end of a cable, without any joining step, so that it may besubsequently fitted onto the external surface of the joint.

With an extrusion operation it is also possible to obtain bodies havinga cross section with any geometrical shape required, for example such asthat illustrated in FIG. 4.

In order to facilitate the extrusion operation, which is per seconventional and therefore not described in detail hereinbelow, saidbodies may be provided with a central, generally metallic core.

The latter performs the function of a support element which is acted on,during extrusion of the expanded polymeric material, by the pullingforce of a pulling device—for example a pulling capstan—designed toreceive the continuous extruded product, prior to the abovementionedoperation involving cutting to size.

This central core may be subsequently removed, leaving a through-hole inthe portion 58 of each body 52, said hole providing said portion with agreater elasticity which is particularly advantageous duringinterconnection of said bodies.

Expansion of the polymeric material is performed during the extrusion ormoulding step and may take place either chemically, through the additionof a suitable expanding agent, i.e. an agent capable of producing a gasunder given pressure and temperature conditions, or physically, by meansof injection of high-pressure gas directly into the extruder cylinder.

Examples of suitable expanding agents are: azodicarbamide,para-toluenesulfonyl hydrazide, mixtures of organic acids (for examplecitric acid) with carbonates and/or bicarbonates (for example sodiumbicarbonate), and the like.

Examples of gases which may be injected under high pressure into theextruder cylinder are: nitrogen, carbon dioxide, air, low-boilinghydrocarbons, for example propane or butane, halogenated hydrocarbons,for example methylene chloride, trichlorofluoromethane,1-chloro-1,1-difluoroethane, and the like, or mixtures thereof.

It has been observed that, for the same extrusion conditions (such asspeed of rotation of the screw, speed of the extrusion line, diameter ofthe extruder head), one of the process variables which influences mostthe degree of expansion is the extrusion temperature. Generally, in thecase of extrusion temperatures lower than 130° C., it is difficult toobtain a sufficient degree of expansion; the extrusion temperature ispreferably at least 140° C., in particular about 180° C. Normally, agreater degree of expansion corresponds to an increase in the extrusiontemperature.

Moreover, it is possible to control to a certain extent the degree ofexpansion of the polymer by adjusting the cooling speed. In fact, bysuitably delaying or anticipating the cooling of the polymer which formsthe expanded coating at the extruder outlet, it is possible to increaseor decrease the degree of expansion of said polymer.

According to the present invention, the degree of expansion may varyfrom 5% to 500%, preferably from 30% to 300%, even more preferablybetween 40% and 150%.

As already mentioned above, the expanded polymeric material may or maynot be cross-linked. The cross-linking is performed, after the extrusionstep or the moulding and expansion step, using known techniques, inparticular by means of heating with a free-radical initiator, forexample an organic peroxide such as dicumyl peroxide. Alternatively, itis possible to perform cross-linking using silanes, which envisages theuse of a polymer belonging to the abovementioned group, in particular apolyolefin, to which silane units comprising at least one hydrolizablegroup, for example trialkoxysilane groups, in particulartrimethoxysilane, have been covalently bonded. The bonding of the silaneunits may be performed by means of a free-radical reaction with silanecompounds, for example methyltriethoxysilane, dimethyl diethoxysilane,vinyl dimethoxysilane, and the like. Cross-linking is conducted in thepresence of water and a cross-linking catalyst, for example an organictitanate or a metallic carboxylate. Dibutyltin dilaurate (DBTL) isparticularly preferred.

The protective coating according to the present invention is applicableto any type of joint, as well as to any type of cable to be joined, beit a cable for conveying or supplying energy or a data transmissioncable or a telecommunications cable, or a cable of the mixedenergy/telecommunications type. In this sense, therefore, the term“conductor” is understood as meaning a conductor of the metallic type,with a circular or segment-like configuration, or comprising opticalfibres or of the mixed electrical/optical type.

EXAMPLE 1

A protective coating according to the present invention, for example ofthe type illustrated in FIG. 4, was manufactured using a polymericmaterial commercially known by the name HIGRAN SD 817® (produced byMontell S.p.A.). This material is a, high melt strength polypropylenemixed with an ethylene/propylene rubber in a percentage ratio by weightof 80/20.

This coating was produced by means of extrusion using an 80 mmsingle-screw extruder in a 25 D configuration, with a speed of rotationof said screw equal to 15 revolutions per minute.

In the extruder and in the extrusion head the thermal profile shown inTable I was used.

TABLE I Extruder zone Temperature (° C.) Screw Neutral Zone 1 150 Zone 2180 Zone 3 200 Zone 4 200 Body 210 Head 200 Mould 200

The temperature of the melt was about 210–220° C.

Expansion of the polymeric material was obtained chemically, by addinginto the hopper of the extruder (by means of a feeder screw controlledby means of a gravimetric metering device), the expanding agentHydrocerol® BIH40 (citric acid/sodium bicarbonate), produced byClariant, in a quantity equal to 1.5% by weight with respect to thepolymer base material.

The protective coating thus obtained had a thickness of about 10 mm.

Said coating was then applied onto the external surface, namely onto theexternal polymeric sheath, of a conventional joint of the single-phaseElaspeed® type, used for joining together a pair of single-poleelectrical cables with a copper conductor having a cross section of 150mm² and an operating voltage of 20 kV.

The total length of the joint was about 800 mm and its external diameterwas about 50 mm. The external diameter of the protective coating wasabout 70 mm.

Impact resistance test

The mechanical impact strength of the abovementioned joint, providedwith the protective coating according to the present invention wasassessed by carrying out impact tests on several zones of the joint,with subsequent evaluation of the damage. This evaluation was carriedout by means of a visual analysis of the joint at each point of impactand by means of measurement of the resistance of the joint insulation.

This test was carried out in accordance with the CENELEC Standard No. HD628 S1 dated December 1995 which envisages the positioning of a joint ona rigid support, in a horizontal position with respect to the latter,namely with the longitudinal axis of this joint parallel to saidsupport. If necessary, sand may be arranged around the joint in order toimpart greater stability to the said joint—in view of the variations incross section which characterize it—during the test.

Before the actual impact test, the resistance of the joint insulationwas measured in accordance with the procedures indicated in theabovementioned standard.

Subsequently an impacting wedge having a V-shaped end with a slightlyrounded form (radius of curvature of 2 mm) was allowed to fall from thesame height (1000 mm) onto three different zones of the joint. Ingreater detail, the impacting wedge was positioned so as to strike bothends of the joint, at the point where the cross section of the saidjoint starts to change and at an intermediate position which is centralwith respect to the joining zone. In order to produce different impactforces (J), impacting wedges with different weights were used.

The joint was then immersed in water for 24 hours and measurement of theresistance of the abovementioned joint insulation was repeated, saidmeasurement having the same value recorded at the start of the test.

At the end of the tests, the protective coating according to the presentinvention was removed in the impact zones and the external polymericsheath and the insulating coating of the joint analysed in order toassess visually the presence or otherwise of any residual deformationdue to the impact of the wedge.

The results of said tests are summarized in Table 2.

Measurement of the Thermal Conductivity

As will emerge more clearly in the continuation of the presentdescription, in order to be able to calculate the thermal resistance ofthe protective coating according to the invention, measurements thethermal conductivity were carried out on the material used.

These measurements were carried out on the basis of the standard ASTM E1530.

At the maximum operating temperature of a joint, equal to about 80° C.,and for an expansion value of the abovementioned material equal to 45%,a thermal conductivity value of 0.11 W/° C.*m was obtained.

Since the thermal resistivity is equal to the inverse of the thermalconductivity, the thermal conductivity value obtained above had acorresponding thermal resistivity value of 9° C.*m/W.

EXAMPLE 2 (COMPARISON)

In a similar manner to that described in Example 1, a single-phaseElaspeed® joint (of the same type as that used in Example 1) wasprovided, in a position radially outside the latter, with a protectivecontainer of the known art.

In greater detail, this container consisted of an aluminium muffle withan internal diameter of 110 mm, filled with a polyurethane resin havinga thermal resistivity, typical of this material, of 6.5 C*m/W. Thethickness of said resin inside said muffle was 30 mm.

This joint was then subjected to impact resistance tests in a similarmanner to that described in Example 1. The results obtained aresummarized in Table 2.

Further below in the present description, the thermal resistance valueof the abovementioned resin layer was calculated and compared with thethermal resistance value of the protective coating according to theinvention.

EXAMPLE 3 (COMPARISON)

Impact resistance tests, as described in Example 1, were also carriedout on a single-phase Elaspeed® joint (of the same type as that used inthe two preceding examples), but without any protective coating. Theresults obtained are shown in Table 2.

TABLE 2 Joint Joint Joint according to according to according to Impactenergy Example 1 Example 2 Example 3 applied (invention) (comparison)(comparison)  40 J No damage No damage Not (weight of acceptable wedge:4 kg)   80 J No damage Slight Not (weight of damage acceptable wedge: 8kg)  120 J No damage Damage at Not (weight of barely performed wedge: 12kg) acceptable limit 160 J Minimal Not Not (weight of externalacceptable performed wedge: 16 kg) damage. No internal damage

The results of the impact tests show that the protective coatingaccording to the invention, for the same impact force applied, ensures amechanical strength greater than or equal to that provided by theprotective devices according to the known art.

More particularly, the results obtained show how a joint provided with aprotective coating according to the invention does not have anystructural damage (namely any damage to the insulating layer) also inthe case of impact force values considerably higher than theacceptability limit of 120 J of joints protected in a conventionalmanner.

Calculation of the Thermal Resistance

It is known that:

$\begin{matrix}{P = {{R_{e}\mspace{14mu} I^{2}} = \frac{\Delta\; T}{R_{tot}}}} & (1)\end{matrix}$where:

-   P is the power supplied by a cable;-   R_(e) is the electrical resistance of the conductor of the cable;-   I is the intensity of electric current flowing in the cable;-   ΔT is the difference between the temperature of the conductor and    the temperature of the ground surrounding the system in question;-   R_(tot) is the total thermal resistance of this system.

From formula (1) the following is obtained:

$\begin{matrix}{I = \sqrt{\frac{\Delta\; T}{R_{e}\mspace{14mu} R_{tot}}}} & (2)\end{matrix}$

Considering that:

-   a) once the type of conductor has been chosen both in terms of    material and in terms of geometry of the cross section, the value of    Re is univocally defined;-   b) assuming T=20° C. for the temperature of the ground and T=90° C.    for the maximum operating temperature of the system, ΔT=70° C.=cost,    the result of formula (2) is that the intensity of current flowing    in the system is greater the smaller the value of R_(tot).

It is known, moreover, that the thermal resistance of a layer made of agiven material is defined as:

$\begin{matrix}{{Rp} = {\rho_{t}\frac{\ln\frac{\phi_{e}}{\phi_{i}}}{2\;\pi}}} & (3)\end{matrix}$where:

-   R_(p) is the thermal resistance of said layer;-   ρ_(t) is the thermal resistivity of the material from which said    layer is made;-   φ_(e) is the external diameter of said layer;-   φ_(i) is the internal diameter of said layer.

Let us assume now that the system considered is a joint provided with aprotective coating, as described in Examples 1 and 2 above.

In this case, the total thermal resistance R_(tot) of the system isdefined by the following parameters:R _(tot) =R _(is) +R _(g) +R _(p) +R _(te)  (4)

where:

-   R_(is) is the thermal resistance of the cable insulating material;-   R_(g) is the thermal resistance of the joint;-   R_(p) is the thermal resistance of the protective coating of said    joint;-   R_(te) is the thermal resistance of the ground.

It is therefore possible to distinguish between the following cases:

-   1) Joint provided with the Protective coating according to Example    1.

Assuming that:

-   φ_(e)=70 mm is the external diameter of the protective coating    according to the invention;-   φ_(i)=50 mm is the internal diameter of said coating, coinciding    with the external diameter of the joint;-   ρ_(t)=9° C.*m/W is the thermal resistivity of the material from    which the protective coating according to the invention is made,

applying the formula (3), the following is obtained:

$\begin{matrix}{{Rp}_{1} = {{9\frac{\ln\;\frac{70}{50}}{2\;\pi}} = {0.48{^\circ}\mspace{14mu}{C.}*{m/W}}}} & \left( 5^{\prime} \right)\end{matrix}$where Rp₁ is the thermal resistance of the protective coating accordingto the present invention.

-   2) Joint provided with the protective coating according to Example    2.

Assuming that:

-   φ_(e)=110 mm is the external diameter of the protective coating    according to the known art;-   φ_(i)=50 mm is the internal diameter of said coating, coinciding    with the external diameter of the joint;-   ρ_(t)=6.5° C.*m/W is the thermal resistivity of the resin forming    the abovementioned protective coating, applying the formula (3), the    following is obtained:

$\begin{matrix}{{Rp}_{2} = {{6.5\frac{\ln\;\frac{110}{50}}{2\;\pi}} = {0.82{^\circ}\mspace{14mu}{C.}*{m/W}}}} & \left( 5^{\prime\prime} \right)\end{matrix}$where Rp₂ is the thermal resistance of the protective coating made ofresin according to the known art.

From the above calculations it is possible to note how the thermalresistance (Rp₁) of the protective coating according to the presentinvention is equal to about half the thermal resistance (Rp₂) of theprotective coating made of resin according to the known art.

Moreover, assuming that:

a) the diameter of the conductor is 20 mm;

b) the external diameter of the cable insulating material is 30 mm;

c) the thermal resistivity of the cable insulating material and thejoint is typically 3.6° C.*m/W,

applying the formula (3) the following is obtained:

$\begin{matrix}{R_{is} = {{3.6\frac{\ln\;\frac{30}{20}}{2\;\pi}} = {0.23{^\circ}\mspace{14mu}{C.}*{m/W}}}} & (6) \\{R_{g} = {{3.6\frac{\ln\;\frac{50}{30}}{2\;\pi}} = {0.3{^\circ}\mspace{14mu}{C.}*{m/W}}}} & (7)\end{matrix}$

Moreover, a typical value for the thermal resistance of the ground is:R _(te)=0.3° C.*m/W  (8)

Applying the formula (4) and taking into account the results (5′), (5″),(6), (7) and (8), the following is obtained:

-   -   a) R_(tot)1.31° C.*m/W using the protective coating according to        the invention, and    -   b) R_(tot)1.65° C.*m/W using the resin coating according to the        known art.

From the above it therefore emerges that, with the protective coatingaccording to the present invention, it is possible to obtain a thermalresistance of the protective coating which is about 20-25% less than thethermal resistance of the protective coating according to the known art(Example 2). On the basis of the abovementioned formula (2) it istherefore possible to note that a current intensity greater than that ofthe known art may be permitted with the protective coating according tothe present invention.

It has to be emphasized how this result has been achieved using aprotective coating which, as illustrated further above with reference toTable 2, provides a mechanical impact strength at least equal to, and insome cases even better than, that of the known art, thereby meaning thatthe reduction in the thermal resistance of the protective coating hasnot been obtained at the cost of a deterioration in the mechanicalproperties of the latter.

Moreover, it must be emphasized also that these advantageous results interms of thermal resistance and mechanical impact strength have beenobtained with a protective coating having a thickness considerably lessthan that of the known art (10 mm in Example 1 compared to 30 mm inExample 2). This means, therefore, that, with the protective coatingaccording to the invention, it will be possible to prepare ajoint/protective coating system having overall dimensions which aredecidedly smaller and with a lower weight, aspects which are regarded asbeing particularly positive in view of the restricted spaces which aretypical of trenches used for laying a cable.

It is necessary, moreover, to underline the fact that the abovementionedexamples refer to a joint with extremely small dimensions. The personskilled in the art may therefore easily understand how the advantageouseffects of the protective coating according to the present invention areeven more obvious the greater the dimensions of the joint considered(for example a joint for three-pole cables or a high-voltage joint).

The present invention offers some major advantages compared to the knownart.

A first advantage consists in the fact that, as already mentioned, theprotective coating according to the invention represents a simpler andmore rapid solution to implement compared to those of the known art.

In fact, as already mentioned, the metal protective containers, whichare widely used as mechanical reinforcements for the joining zone of twocables involve: 1) assembly difficulties, which are all the greater inthat the operations are performed in narrow and uncontrolledenvironments such as laying trenches; 2) the need for particularlydelicate and complex additional operations such as the introduction of afilling material inside the container; 3) problems of toxicity andhandling of said filling material in the case where epoxy andpolyurethane resins are used; 4) long assembly times; 5)difficulty—owing to the rigidity of the container—of adaptation to theexternal profile of the joint; 6) intense use of specialized labour.

The present invention, on the other hand, is able to overcome thesedisadvantages owing to a greater simplicity of use due to both theelimination of the abovementioned filling material and the greater easeof handling of the invention, also as a result of the limited weightthereof. It can be clearly understood that the elimination of theabovementioned disadvantages results in a considerable reduction incosts, installation time and difficulties for personnel working insidethe trenches.

Further advantages of the protective coating according to the presentinvention consist in the high mechanical strength which said coating isable to ensure, while permitting a thickness—and therefore the overalldimensions of the protective joint—which are particularly small and anexcellent heat transfer between joint and external environment.

The abovementioned protective coating has, in fact, a high capacity forabsorbing impacts, reducing considerably the impact force actuallytransferred to the underlying joint, in particular to the insulatingcoating of the latter. Owing to this high absorption capacity,therefore, it is possible to reduce considerably the thickness of thecoating, resulting in the advantage of smaller overall dimensions of thejoint and easier handling and simpler installation of the said coating.Moreover, a reduction in this thickness also results in a particularlyadvantageous aspect in terms of the heat exchange between joint andexternal environment since, as already mentioned, the thermal resistanceis directly proportional to the thickness.

With the coating according to the present invention, moreover, it ispossible to provide a protective layer of the continuous type which,unlike the known art, is able to ensure a mechanical impact strengthover the whole external surface of the joint without giving rise to theformation of portions which are unprotected or partially protected and,therefore, potentially liable to damage.

1. A joint for electrical cables designed to convey or supply energy,said joint comprising: at least one electrical connection between aconductor of a first electrical cable and a conductor of a secondelectrical cable; at least one electrical insulating layer arranged in aposition radially outside said connection; and a protective coatingarranged in a position radially outside said electrical insulatinglayer, said coating comprising an expanded polymeric material suitablefor providing said connection with a mechanical impact strength and, atthe same time, ensuring a predetermined heat exchange between saidconnection and the external environment, wherein said protective coatingcomprises a plurality of modular components arranged around saidconnection, said modular components being connected to one another attheir respective edges such that said protective coating is axially andcircumferentially continuous with respect to said connection, whereinfurther said modular components enable the circumference of saidprotective coating to be adjusted to fit around joints of varying sizethrough the addition or removal of one or more of said modularcomponents.
 2. The joint according to claim 1, wherein said heatexchange ensures that said connection operates at a temperature lessthan the maximum operating temperature of said connection.
 3. The jointaccording to claim 2, wherein the maximum operating temperature is equalto about 80° C.
 4. The joint according to claim 1, wherein said expandedpolymeric material is a polyolefinic polymer or copolymer based onethylene or propylene.
 5. The joint according to claim 1, wherein saidexpanded polymeric material has a degree of expansion betweenapproximately 5% and approximately 500%.
 6. The joint according to claim5, wherein the degree of expansion is between 30% and 300%.
 7. The jointaccording to claim 6, wherein the degree of expansion is between 40% and150%.
 8. The joint according to claim 1, wherein the expanded polymericmaterial has a thickness of between 3 mm and 25 mm.
 9. The jointaccording to claim 8, wherein the thickness is between 3mm and 15mm. 10.The joint according to claim 1, wherein said expanded polymeric materialis an extruded or molded polymeric material expanded in the presence ofan expanding agent.
 11. The joint according to claim 10, wherein saidexpanding agent is a high-pressure gas.
 12. The joint according to claim1, wherein said expanded polymeric material is cross-linked.
 13. Thejoint according to claim 1, wherein each of said plurality of modularcomponents is elongated and comprises a substantially Y-shapedcross-section, wherein a base of said Y-shaped cross-section of a firstof said modular components is adapted to mate with a diverging portionof said Y-shaped cross-section of a second of said modular components.14. A joint for electrical cables designed to convey or supply energy,said joint comprising: at least one electrical connection between aconductor of a first electrical cable and a conductor of a secondelectrical cable; at least one electrical insulating layer arranged in aposition radially outside said connection; and a protective coatingarranged in a position radially outside said electrical insulatinglayer, said coating comprising an expanded polymeric material suitablefor providing said connection with a mechanical impact strength and, atthe same time, ensuring a predetermined heat exchange between saidconnection and the external environment, wherein said protective coatingcomprises a plurality of modular components, said modular componentsbeing linked together at joints such that said protective coating isaxially and circumferentially continuous with respect to saidconnection, said modular components being partially free to sliderelative to one another at said joints without said protective coatingbecoming axially or circumferentially discontinuous.
 15. The jointaccording to claim 14, wherein said heat exchange ensures that saidconnection operates at a temperature less than the maximum operatingtemperature of said connection.
 16. The joint according to claim 15,wherein the maximum operating temperature is equal to about 80° C. 17.The joint according to claim 14, wherein said expanded polymericmaterial is an extruded or molded polymeric material expanded in thepresence of an expanding agent.
 18. The joint according to claim 14,wherein said expanded polymeric material has a degree of expansionbetween approximately 5% and approximately 500%.
 19. The joint accordingto claim 14, wherein the expanded polymeric material has a thickness ofbetween 3 mm and 25 mm.
 20. The joint according to claim 14, whereineach of said plurality of modular components is elongated and comprisesa substantially Y-shaped cross-section, wherein a base of said Y-shapedcross-section of a first of said modular components is adapted to matewith a diverging portion of said Y-shaped cross-section of a second ofsaid modular components.